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#myosin

13 public questions tagged with this topic.

Cytoskeleton

Which statement is TRUE about myosin and muscle contraction?

Among eighteen myosin classes class II myosins dominate muscle mechanics due to capacity to assemble into higher order structures essential for force scaling. Each myosin II molecule comprises two heavy chains with N terminal motor domains hydrolyzing ATP IQ motifs binding essential and regulatory light chains forming lever and C terminal extended coiled coil driving dimerization plus further self association into filaments. In skeletal muscle dimers polymerize antiparallel into 1.6 micron long thick filaments containing about 300 molecules bare zone in middle heads projecting helically outward 14.3 nm periodicity. These interdigitate with thin actin filaments decorated with troponin tropomyosin facilitating sliding filament mechanism with ordered cross bridge cycling. Non muscle myosin II isoforms form smaller filaments for cortical tension and adhesion. Myosin V and VI transport cargo on actin toward plus and minus ends myosin I tethers membranes myosin X bundles filopodia none form stable contractile thick filaments in sarcomeres. Therefore skeletal and cardiac contractility fundamentally depends on myosin II filament assembly and ATP driven power stroke scaled by light chain phosphorylation.

Ref: Lodish et al., Molecular Cell Biology, Chapter 18 – Myosin II major myosin of muscle thick filaments.

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Cytoskeleton

Which type of myosin is primarily involved in non-muscle cell contractility?

Non muscle contractility underpins stress fiber maintenance, adhesion maturation, morphogenesis and wound closure, distinct from skeletal muscle sarcomeres employing troponin regulation. Myosin II family in non muscle cells includes isoforms IIA, IIB and IIC sharing hexameric composition two heavy chains plus essential and regulatory light chains, forming short bipolar filaments that transiently associate with actin rather than stable thick filaments. Activation occurs via phosphorylation of 20 kilodalton regulatory light chain RLC at Ser19 by calcium calmodulin dependent MLCK and Rho associated kinase ROCK, converting autoinhibited 10S folded conformation to extended 6S assembly competent state enhancing actin activated ATPase. Coordinated pulling of antiparallel actin bundles generates isometric tension, retrograde flow and cortex stiffness. Isoform IIA drives rapid adhesion turnover at leading edge, IIB sustains prolonged tension at rear. Myosin I single headed tension sensor, myosin III adaptation in photoreceptors, myosin VI minus end directed for endocytosis, not primary contractile generator. Therefore myosin II remains central for non muscle contractility.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 18: Nonmuscle Myosin II in Contractility.

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Cytoskeleton

What happens when ATP is hydrolyzed by myosin in the actin-myosin cycle?

Actomyosin ATPase cycle illustrates chemomechanical coupling. Starting rigor actin myosin complex with empty nucleotide pocket tightly bound, ATP binding induces opening of actin binding cleft and dissociation rate 1000 per second. Free myosin closes Switch I Switch II around gamma phosphate hydrolyzing ATP to ADP inorganic phosphate with rate 50 per second, coupled to recovery stroke where lever arm swung 90 degrees from post stroke to pre stroke position, converter domain rotation storing elastic energy, myosin primed high energy configuration. This hydrolysis induced conformational change essential; without hydrolysis myosin would remain unprimed unable to generate force upon rebinding. Myosin ADP Pi then weakly attaches actin, strong binding and phosphate release drive power stroke, ADP release returns rigor. Actin filament does not depolymerize during cycle, actin stable scaffold, ADP release separate step after power stroke. Therefore ATP hydrolysis drives myosin head undergoing conformational change cocking lever arm into energized state preparing for phosphate gated power stroke that performs mechanical work during contraction.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Myosin ATP Hydrolysis and Conformational Change.

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Cytoskeleton

The major function of titin in muscle contraction is to:

Massive protein titin, encoded by TTN gene 363 exons, 38,138 residues in canonical cardiac isoform N2B 3.8 megadalton, spans half sarcomere from Z disc to M line, single molecule. N terminal Z disc anchor binds alpha actinin via Z repeats 1 to 7 and telethonin T cap forming antiparallel complex, I band extensible region contains tandem immunoglobulin like domains Ig 80 repeats that unfold at low force providing entropic elasticity, PEVK region rich in proline glutamate valine lysine behaving as worm like chain, and cardiac specific N2B element with spring properties. A band region with FN3 and Ig super repeats binds myosin thick filament and myosin binding protein C regulating assembly spacing, C terminal M line segment interacts with myomesin maintaining thick filament centrality. Passive tension generated upon stretch restores resting length, contributes to diastolic filling and Frank Starling length dependent activation. Titin does not cap actin, not motor hydrolyzing ATP, not activate myosin, function elastic scaffold binding myosin and providing reversible extensibility central to muscle mechanics and sarcomere stability.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: Titin Elasticity and Myosin Binding.

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Cytoskeleton

Which of the following determines the speed of myosin movement along actin filaments?

Mechanistic model of myosin step size explains velocity differences among isoforms. Lever arm hypothesis states small conformational change of converter domain upon phosphate release rotation about 10 degrees amplified by rigid lever arm helical neck decorated with IQ motifs bound to light chains acting as fulcrum. Step size roughly equals lever length times angular change, so each IQ motif about 5 to 6 nanometer contributes. Constructing chimeras fusing artificial alpha actinin repeats or duplicating IQ motifs increased sliding speed linearly in motility assays without altering ATPase kcat, confirming length determines speed. Myosin V with six IQ motifs long lever moves 36 nanometer step fast cargo transport, myosin II with two IQ short lever 5 to 10 nanometer slower but ensemble force generation. Regulatory light chain phosphorylation changes recruitment and duty ratio in smooth muscle, actin length affects number of interacting heads not intrinsic speed, ATP concentration alters velocity only below Km about 50 micromolar. Therefore intrinsic determinant of maximal speed at saturating ATP is length of lever arm translating small catalytic domain rotation into amplified translation and faster sliding.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Lever Arm Length and Myosin Speed.

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Cytoskeleton

The power stroke in the actin-myosin cycle occurs due to:

Cross bridge cycle parsed by transient kinetics reveals distinct substeps responsible for detachment priming and force generation. Myosin rigor complex actin myosin no nucleotide strongly bound. ATP binding rate 1 to 3 per micromolar per second causes rapid dissociation 500 to 1000 per second. Detached myosin hydrolyzes ATP to ADP plus inorganic phosphate rate about 50 per second inducing recovery stroke moving lever arm into pre power stroke high energy conformation approximately 90 degree rotation storing strain. Myosin ADP Pi weakly binds actin via electrostatic contacts. Isomerization into strongly bound state triggers phosphate release from active site, observed as burst of Pi, accompanied by closure of actin binding cleft and 60 degree rotation of converter domain generating power stroke dragging actin 5 to 10 nanometer generating 2 to 6 piconewton force. ADP remains bound temporarily then released rate limiting step about 20 per second returning to rigor. Thus power stroke specifically driven by phosphate release, not ATP binding which detaches, nor ADP binding which slows, nor actin monomer exchange unrelated to motor mechanism.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 18: Power Stroke and Phosphate Release Mechanism.

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Cytoskeleton

Which myosin type is responsible for intracellular cargo transport?

Intracellular cargo trafficking covers both microtubule and actin routes. Long range 10 to 100 micron interphase transport uses kinesin dynein on microtubules, while short range local transport within cortical actin rich zones 1 to 10 micron uses myosin. Processive myosin capable of multiple steps without dissociation required for efficient transport. Myosin V class features high duty ratio spending about 70 percent cycle strongly attached, dimeric coiled coil tail, six IQ motifs with calmodulin light chains per head forming long 24 nanometer lever arm achieving 36 nanometer step matching helical repeat of actin, hand over hand walking mechanism with gating coordinated by intramolecular strain. Transport cargos include melanosomes via interaction with melanophilin Rab27a in melanocytes, synaptic vesicles, endoplasmic reticulum and mRNA in budding yeast via Myo2 Myo4. Myosin I monomeric membrane tether, myosin II bipolar contractile filaments for cytokinesis low duty ratio non processive, myosin VI minus end directed for endocytosis. Therefore myosin V primary dedicated motor for processive intracellular cargo transport along actin cables and cortical networks.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: Myosin V and Cargo Transport.

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Cytoskeleton

Actin filaments interact with which motor protein for intracellular transport?

Two distinct superfamilies of cytoskeletal motors evolved to walk along filaments using ATP hydrolysis. Myosin motors share N terminal head motor domain containing actin binding interface including helix turn helix, cardiomyopathy loop, P loop ATPase, Switch I II for gamma phosphate sensing and lever arm with IQ motifs binding calmodulin light chains. Upon interaction with F actin 7 nanometer helical filament, head undergoes conformational cycle releasing phosphate driving swing of lever arm producing force along filament. Diverse isoforms specialize: myosin II non processive forming bipolar filaments for contraction, myosin V processive dimer for cargo, myosin I single headed tension sensor. In contrast dynein heavy chain AAA plus ring and coiled coil stalk binds microtubule 25 nanometer tubule, kinesin motor domain with tubulin binding loops moves along protofilaments. Nexin is non motor linker between doublet microtubules in axoneme. Therefore actin filament based intracellular transport and contractility specifically require myosin family as dedicated actin associated motor providing directional movement.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Myosin Motors and Actin Filaments.

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